1. Field of the Invention
This invention relates to producing, without sacrificing pulp strength and yield, unbleached kraft pulps with minimal residual lignin to reduce the consumption of bleaching chemicals.
2. Description of the Prior Art
In the conventional batch cooking process, a digester is filled with wood chips and charged with a cooking liquor, which in the kraft process is an aqueous solution of sodium hydroxide and sodium sulfide. The digester is then sealed, and heated to cooking temperature by direct or indirect heating with steam. At the end of the cook, the pulp is discharged through a blow valve. Because all the pulping chemicals are charged at or before commencement of the cook, the degradation and removal of carbohydrates in the highly alkaline initial pulping phase tends to be accelerated. In such a process, the delignification rage and selectivity are strongly decreased after about one third of the cooking time because the effective alkali concentration decreases to about one third of the initial concentration, and lignin concentration in solution becomes increasingly higher.
In the mid-1980s, liquor displacement in kraft batch digester operations was introduced. Heat economy was the original driving force behind the development of this technology, as described in U.S. Pat. No. 4,601,787. This technique provided the possibility of extended delignification, and has been named by Beloit Corp. as the RDH.RTM. system as described by E. K. Andrews in 1989 TAPPI Pulping Conf., 1989, 607-625, and by Sunds Defibrator as the Super-Batch.RTM. system as described by S. Pursiainen TAPPI, 73(8), 1990, 115-122. It is generally suggested that the benefits in improved pulp strength delivery and the extent of delignification in liquor displacement technology are achieved by: (1) the high initial sulfide concentration resulting from warm and hot black liquor impregnation (Mera and Chamberlin, TAPPI, 71(1) 1988 132-136; Tormund and Teder, Int. Symp. Wood & Pulping Chem. 1989, 247-254), (2) the uniform chemical and temperature distribution in the digester during the cooking phase (Tikka and Kovasin CCPA Spring Conf. 1990, 1-9), and (3) the utilization of a modified effective alkali concentration profile during the different phases of cooking (Pu et al., APPATI, 44(6) 1991, p. 399). Further modification of this type of process has been proposed in U.S. Pat. No. 4,849,052 as: after the black liquor impregnation stage(s) the cooking stage is split into multi-cooking stages (generally 3.+-.1 stages) to provide more even distribution of EA concentration, which in concept is similar to that used in Kamyr MCC and EMCC operations. The major drawback of these liquor displacement processes is the heavy capital investment.
Processes including a black liquor treatment stage before addition of white liquor were proposed to save cooking chemicals and achieve faster delignification rate several decades ago (U.S. Pat. No. 2,639,987), and investigated in detail recently by Engstrom et al (Paperi Ja Puu, 76(1), 1994, p. 59). Faster delignification was generally obtained after the black liquor pretreatment of wood chips. Air dried wood chips, however, were used in these processes. The results may not be valid for the wood chips used in pulp mills, which are generally wet, and contain 30-50% moisture. Further work suggests that when wet wood chips are used in this type of processes faster delignification is obtained, but no improvement of pulping selectivity or pulp viscosity is obtained. In addition, because no new cooking chemical is added to the black liquor, and the treatment time is relatively short, lignin removal during the black liquor treatment is minimal. The lignin concentration in the cooking liquor during the cooking period is about the same as that without black liquor pretreatment.
The most recent development in liquor displacement processes is the ENERBATCH.RTM. process (Wizani, 1992, TAPPI Pulping Conf. 1037-1046), which impregnates the wood chips with a strong white liquor under pressure to provide uniform chemical distribution in the chips.
In continuous cooking, a recent advance has been the Kamyr extended modified continuous cooking (EMCC.RTM.) process. Enhanced pulping selectivity has been attained by prolonged low-temperature counter-current cooking (Jiang et al., APPITA 1991, p. 221). The major changes from MCC.RTM. to EMCC.RTM. are reduced cooking zone temperature, increased washing zone temperature, and addition of white liquor to the washing zone. It has been suggested that the better , selectivity in this process is a result of the lower lignin concentration and temperature during cooking.